This invention relates to systems and methods for ensuring that a single refrigeration unit supplying refrigerant for a number of individual control channels is not overtaxed by the demands of one or more of the channels.
In a number of situations in which controllable refrigeration capacity is needed, it is advantageous to utilize a single refrigeration unit for providing pressurized refrigerant to two or more control channels, each of which is separately regulated to maintain the temperature of an existing fluid circuit or device. An example is found in the above referenced prior filed application, which is particularly described in terms of an example for controlling the temperature of a number of different tools in a cluster tool system for fabricating semiconductors. In this context, minimal floor space and high reliability are of paramount importance, so that a compact physical configuration, long term operation, and precise and stable temperature control are significant operative goals. The system described and claimed in the referenced application achieves these objectives by utilizing a single large refrigeration unit having adequate nominal capacity for total demand and providing pressurized liquid refrigerant into separate control channels, each including a control loop for adjusting the temperature of heat transfer fluid circulated through the associated tool. The flow rate of the refrigerant in each channel is separately controlled in accordance with the target temperature needed at the tool at particular point in time. In each channel, the refrigerant is fed at its chosen rate, and in a two phase state, into an evaporator/heat exchanger thus being at the evaporation temperature. In the heat exchanger heat of evaporation is given up in cooling a thermal transfer fluid also passing through the evaporator/heat exchanger at a constant rate. This occurs independently for each of the channels, which can have different refrigeration demands at any particular time. For example, a maximum demand can be imposed in one channel which is required to drop the tool temperature as rapidly as possible, as to effect a process changeover, while one or more other channels are, temporarily at least, in a relatively stable mode.
In this system, the temperature level that is to be regulated is that of the semiconductor fabrication tool, which is outside the closed refrigeration loop. Thus the superheat (the difference between refrigerant temperature before and after heat exchange) cannot be used for control and limiting of demand.
Under these circumstances, an unstable condition can arise when any channel or the refrigeration unit itself is temporarily overtaxed, since the flow rate to each evaporator is servoed to the target temperature of the remote tool. The instability arises when the return refrigerant is not entirely in the gas phase, so that liquid is present in the refrigerant returned to the compressor from one or more channels. In consequence, a well known but complex reaction can occur within the refrigeration system in which the compressor drops in efficiency (and can be damaged). When this "flooding" occurs, the greater the demand for cooling capacity the lower will be the performance (the opposite of what is wanted) and, as flow is increased the more liquid will be returned, increasing the likelihood of damage to the compressor system. It is not feasible in this type of system to place arbitrary limits on the refrigeration capacity of each channel, because this would unduly extend the costly process times involved in semiconductor fabrication, even though adequate refrigeration capacity may be available for each channel.
Although conventional proportional flow valves, including temperature responsive control valves using bimetallic elements, can be utilized in the separate channels, it is preferred, primarily because of higher reliability, but also for reasons of linearity, resolution and freedom from hysteresis to employ thermal expansion valves which are responsive to the pressure in a closed gas circuit. These closed circuits include a bulb containing a pressurized gas that is positioned proximate a temperature source at a given temperature level, such as a chilled refrigerant conduit. A tube from the bulb communicates with a volume, within the valve, that is bounded by a flexible diaphragm, which then flexes in response to the gas pressure. The diaphragm in turn controls the position of a valve element which occludes a flow orifice to a selected degree. The internal pressure within the bulb and the closed pressure circuit can be changed by command signals applied to a heater in thermal interchange relation with the bulb, and regulated by a command servo circuit As described in a previously filed application of Richard Petrulio, et al. Expansion Valve Unit, Ser. No. 08/953,101, filed Oct. 17, 1997, assigned to the assignee of the present invention, this system can incorporate a thermal insulation between the heater and the bulb in order to integrate temperature fluctuations. It can be used so as to control refrigerant flow in response to a remotely detected temperature (i.e. a tool, controlled unit, or refrigerated compartment).
Whether referred to as regulation of capacity or avoidance of flooding of the compressor, the system must avoid initiating the astable condition, while at the same time it must be able to utilize the available refrigeration capacity in the most efficient manner for each of the two or more independently operable channels that may be used in the system. Since the control temperatures and the refrigeration demands in individual channels vary independently and since total refrigeration rate also changes, these objectives present unique problems if overtaxing the refrigeration unit is to be avoided.